1. Field of the Invention
This invention relates to an apparatus for a continuous production of a crystalline zeolite material, and more particularly to an apparatus for a continuous up-flow crystalline zeolite material production.
2. Description of the Prior Art
Zeolitic materials, both natural and synthetic, have been demonstrated in the past to have catalytic properties for various types of hydrocarbon conversions. Certain zeolitic materials are ordered, porous crystalline aluminosilicates having a definite crystalline structure within which there are a large number of channels. These cavities and channels are precisely uniform in size. Since the dimensions of these pores are such as to accept for adsorption molecules of certain dimensions while rejecting those of larger dimensions, these materials have come to be known as "molecular sieves" and are utilized in a variety of ways to take advantage of these properties.
Such molecular sieves, both natural and synthetic, include a wide variety of positive ion-containing crystalline aluminosilicates. These aluminosilicates can be described as a rigid three-dimensional framework of SiO.sub.4 and AlO.sub.4 in which the tetrahedra are cross-linked by the sharing of oxygen atoms whereby the ratio of the total aluminum and silicon atoms to oxygen is 1:2. The electrovalence of the tetrahedra containing aluminum is balanced by the inclusion in the crystal of a cation, for example, an alkali metal or an alkaline earth metal cation. This can be expressed wherein the ratio of aluminum to the number of various cations, such as Ca/2, Sr/2, Na, K, or Li is equal to unity. One type of cation may often be exchanged either entirely or partially by another type of cation utilizing ion exchange techniques in a conventional manner. By means of such cation exchange, it has been possible to vary the properties of a given aluminosilicate by suitable selection of the cation. The spaces between the tetrahedra are usually occupied by molecules of water prior to dehydration.
Prior art techniques have resulted in the formation of a great variety of synthetic aluminosilicates. These aluminosilicates have come to be designated by letter or other convenient symbols, e.g., zeolite A (U.S. Pat. No. 2,882,243), zeolite X (U.S. Pat. No. 2,882,244), zeolite Y (U.S. Pat. No. 3,130,007), zeolite ZK-5 (U.S. Pat. No. 3,247,195), zeolite ZK-4 (U.S. Pat. No. 3,314,752), zeolite ZSM-5 (U.S. Pat. No. 3,702,886), zeolite ZSM-11 (U.S. Pat. No. 3,709,979), zeolite ZSM-12 (U.S. Pat. No. 3,832,449), zeolite ZSM-20 (U.S. Pat. No. 3,972,983), zeolite ZSM-35 (U.S. Pat. No. 4,016,245), zeolite ZSM-21 and 38 (U.S. Pat. No. 4,046,859), zeolite ZSM-23 (U.S. Pat. No. 4,076,842), and ZSM-48 (U.S. patent applications, Ser. No. 013,640, filed Feb. 21, 1979 and Ser. No. 063,230, filed Aug. 3, 1979). The entire contents of all of these patents and applications are incorporated herein by reference.
Zeolite crystallization is commonly conducted batchwise in large autoclaves, either static or stirred. It frequently requires many hours for completion, and can be, by petroleum industry standards, labor-intensive. Manufacturing processes for commercial zeolites can be classified into two groups, those using homogeneous or heterogeneous hydrogels and those based on pre-formed gels, for example, pelletized gels. The hydrogel processes conventionally employ large vats or autoclaves for stepwise mixing, gel aging and final crystallization, and several examples are reviewed in Chapter 9 of "Zeolite Molecular Sieves", D. W. Breck, John Wiley and Sons (1974). Although it is possible to crystallize some of these zeolites in a continuous-stream process, (as claimed, for example, in Belgian Pat. No. 869,156, July 20, 1978), stepwise, batch processes have heretofore been preferred due to the meta-stability of zeolite species and to the tendency towards nucleation and growth of undesirable zeolite phases such as P, sodalite, and analcime in the cases of zeolites X and Y, zeolite A, and mordenite, respectively, both of which are promoted by a relatively long residence time of the reactants and the products in batch systems.
In contrast, in a continuous up-flow reactor design of the present invention, meta-stability (i.e., stability of less stable zeolite species formed during the synthesis of the product but not necessarily present in the final product or product mixture) and the tendency towards nucleation of undesirable zeolite phases is very effectively controlled by easily adjustable flow rates of the reactant streams, by the stirrer design and by easily adjustable rate of the stirrer rotation.
Many prior art zeolite preparations, in addition to the frequent occurrence of competing zeolite product phases, have been characterized by a zeolite product of SiO.sub.2 /Al.sub.2 O.sub.3 ratio .ltoreq.10 and a high alkalinity, with pH exceeding 12 and often exceeding 14. Expressed in terms of OH/SiO.sub.2 mole ratios, the alkalinity of typical reaction mixture compositions used in the production of zeolites A, X and Y are given by D. W. Breck on page 274, of the aforementioned book as 2, 2.4 and 0.8, respectively.
In contrast, the zeolites prepared by using the apparatus of the present invention form a class of zeolites characterized in their preparation by reaction mixtures of lower alkalinity and by a zeolite product of SiO.sub.2 /Al.sub.2 O.sub.3 &gt;12. In addition, they have a constraint index of between 1 and 12, and they are generally prepared in the presence of a nitrogen- (N) or phosphorus- (P) containing organic compound. For a further description of that method and zeolites produced thereby on a continuous-stream basis, see copending U.S. application Ser. No. 47,538, filed June 11, 1979, whose entire contents are incorporated herein by reference.
As can be seen from the aforementioned copending U.S. Application, the zeolites made by the present inventive apparatus are prepared at OH/SiO.sub.2 mole ratios below 1.0 and often below 0.5. The combination of low OH/SiO.sub.2 and highly siliceous reaction mixtures results in gels which are quite stiff and difficult to mix. It is with these reaction mixtures that a continuous-stream crystallization process affords unique advantages, both in terms of production efficiency and in terms of product quality control. Possible advantages of a continuous-stream process include facile and independent control of nucleation and of growth stages of crystal formation by such techniques as temperature and pH gradients, by staged injection of nutrients such as SiO.sub.2 and Al.sub.2 O.sub.3 source materials and of crystallization modifiers such as N- or P-containing organic compounds, alkali metal salts, acids and bases, and by seeding.
A downflow apparatus for continuous production of zeolites is disclosed in a copending U.S. application, Ser. No. 220,556, filed Dec. 29, 1980, by E. W. Valyocsik. The Valyocsik apparatus is peculiarly adapted to a continuous downflow zeolite synthesis. For example, the stirrer blades are disposed about the vertical shaft of the stirrer at an angle of 0.degree. to 60.degree. and most preferably at an angle of 30.degree. to 45.degree. from the vertical in a downward direction to push the reactants downwardly. The internal surface of the Valyocsik's preferably tubular reactor is smooth to facilitate unobstructed downward flow of the reactants and the products.
While the Valyocsik's downflow reactor configuration may be preferable for synthesis of some zeolites under certain reaction conditions, (e.g., for reaction mixtures where seeding is not important), the up-flow reactor of the present invention is particularly desirable for synthesis of zeolites where seeding appears to play a significant role. In the upflow reactor of the present invention, the fresh reactants may enter through a mass of previously-formed product zeolite crystals which may function as seeds in systems where seeding can induce rapid zeolite nucleation and growth.
Accordingly, it is a primary object of the present invention to provide an improved outflow continuous crystallization apparatus.
It is an additional object of the present invention to provide a continuous upflow apparatus for zeolite synthesis which produces zeolites at substantially improved yields and rates of production than conventional batch apparatus.
Additional objects of this invention will become apparent to those skilled in the art from the study of the specification and the appended claims.